A signal includes informational content and characteristics other than informational content. For example, an optical signal may have an optical power level, which is a measure of the energy delivered by the optical signal per unit of time. In another example, an electrical signal may have a signal-to-noise ratio, which reflects the relative amounts of signal and noise in a transmission. These signal characteristics that do not represent the informational content of the signal are referred to as parametric information, and may be represented by data, called parametric data. The informational content of the signal may be represented by informational content data.
The informational content data and the parametric data may be transmissible at frequencies, or data rates, that are not supported by some circuit elements.